Murine megakaryocyte precursor cells giving rise to platelet-shedding thrombocytes will be quantitated and characterized, and factors (thrombopoietin and feedback inhibitors) regulating these cell populations will be investigated using short-term (semisolid agar) and long-term (liquid suspension) tissue culture systems. The semisolid agar culture assay allows precursor cells (i) to be quantitated; (ii) to determine their cell cycle characteristics; (iii) to assess direct stimulators and potentiators of thrombopoiesis; (iv) to investigate clonal variation in ploidy values and platelet production from single precursor cells, and (v) to examine the relative role of stimulators in megakaryocyte formation and platelet shedding. The long-term culture procedure allows a kinetic approach in a system in which trafficking is not a problem. Regulatory processes can be analyzed by the addition of candidate factors either to induce a wave of megakaryopoiesis or to modulate continuing thrombopoiesis or to shut down differentiation. Accordingly, using these in vitro systems together with presently established in vivo assays, much can be learned of the regulatory mechanisms which influence normal and perturbed thrombopoiesis.